[PATCH 0/3 v3] delaytop: add delay max, timestamp and sorting for top latency analysis

[wang.yaxin]

From: Wang Yaxin <wang.yaxin@xxxxxxxxxx>

Previously delaytop only showed average delays. This patch adds:

1. delay_max fields to track the maximum delay value for each delay type
   (cpu, blkio, irq, swapin, freepages, thrashing, compact, wpcopy)
   per task.

2. The -t/--type option displays only the specified delay type with avg/max
   values side by side, allowing focused analysis:
     delaytop -t cpu    # Show only CPU delay with avg/max
     delaytop -t wpcopy # Show Copy-on-Write delay with avg/max

3. Wall-clock timestamp when each maximum delay occurred, displayed in the
   MAX_TIMESTAMP column when using -t/--type option. This enables:
   - Identifying the time when a process experienced an abnormal delay max
   - Correlating delay max across multiple processes at the same timestamp
   - Cross-referencing with logs, traces, or other metrics at that time

4. When using -t/--type option, tasks are sorted by maximum delay value in
   descending order (largest delay first), enabling quick identification of
   top N processes with highest delay spikes.

Signed-off-by: Wang Yaxin <wang.yaxin@xxxxxxxxxx>

v2->v3:
[patch 1/3]
1. fix get_field_by_name() MODE_TYPE filter breaking --sort for non-type fields
2. zero-init stats struct and cap memcpy size in netlink response parsing
3. fix compare_tasks() count field read as unsigned long long for 32-bit safety
4. strcmp → offset-based access via field_desc.max_offset

[patch 2/3]
1. fix 32-bit time_t overflow check to use max representable value instead of (time_t)-1
2. fix gmtime() pointer aliasing by assigning timespec64 tv_sec to local time_t
3. dropped pre-2000 check; gmtime → localtime_r + stack struct tm

[patch 3/3]
1. fixed Y2038 overflow false negative in format_kernel_timespec()
2. switched to localtime_r() with stack `struct tm` for thread safety.
3. `field_delay_max_and_ts()` `max_ts` nullable; removed unused ts_scratch
4. `-t/--type` mode: hide `[o]sort` and `[M]memverbose`, show only `[q]quit`

Wang Yaxin (3):
  delaytop: add delay max for delaytop
  delaytop: add timestamp of delay max
  delaytop: sort by max delay to highlight top latency processes

 Documentation/accounting/delay-accounting.rst |  43 +++
 tools/accounting/delaytop.c                   | 286 +++++++++++++++---
 tools/accounting/getdelays.c                  |   5 +
 3 files changed, 296 insertions(+), 38 deletions(-)

-- 
2.25.1